This invention relates to methods, apparatuses and an imaging mode for tomographic imaging, particularly for medical x-ray imaging, according to the preambles of the accompanying independent claims.
Imaging methods utilizing electromagnetic radiation can be divided into two groups: radioscopic methods and tomographic methods. in traditional radioscopy the radiation source, the object to be imaged and the radiation detector, e.g. an x-ray film, are stationary with respect to one another during the imaging session. Imaging methods in which a narrow beam is moved over the object to be imaged are also known.
Tomographic methods can be divided into linear (i.e. planar) tomographic methods and complex motion or spiral tomography methods. In tomographic imaging the object to be imaged and/or the radiation detector are moved with respect to each other in a controlled manner, and thus in linear tomography the tomographic movement occurs with respect to one axis and in complex motion tomography with respect to two axes. These methods use a beam which is of the same size as the object to be imaged, and the object is usually held in place as the radiation source and radiation detector are moved dependently on each other on the opposite sides of the object to be imaged in the opposite directions so that the beam penetrates the object from different directions, but its centre of movement/rotation in the object does not move. The methods provide accurate images of the imaging area in the centre of rotation of the beam, whereas the other parts of the object are blurred partially or totally.
There are also xe2x80x98narrow beam tomographyxe2x80x99 methods in which a beam considerably narrower than the object to be imaged sweeps across the area to be imaged and the beam is turned with respect to the object to be imaged. In that case the imaging means (radiation source and radiation detector) must be moved in a controlled manner so that the detector moves in relation to the beam at a lateral velocity which corresponds to the perpendicular sweeping speed of the beam in the area to be imaged multiplied by the ratio of magnification, i.e. by a coefficient which is the ratio of the distance of the beam focus (=radiation source) and the distance of the focus from the area to be imaged. Here the term detector refers to a film or the like; in digital imaging, for example, the movement of the detector with respect to the area to be imaged may be replaced with a suitable electrical function, such as charge transfer on the surface of a semiconductor sensor.
Tomographic methods are commonly used in different fields of medicine, e.g. in odontology. Linear and spiral tomography methods have been employed e.g. for cross-sectional imaging of the dental arc, and narrow beam tomography e.g. for producing panoramic images of the dental arc. Here a panoramic image refers to imaging of the dental arc onto a plane by moving the imaging area of imaging means along a path resembling the shape of the dental arc and keeping the imaging means substantially perpendicular to the dental arc during the whole imaging session.
Prior art tomographic solutions are described e.g. in U.S. Pat. Nos. 4,039,837; 4,985,907; 5,012,501 (priority FI 894339) and U.S. Pat. No. 5,371,775 (priority FI 922361). It is known to produce a tomographic effect by turning the imaging means in a certain direction and by simultaneously moving them with respect to the area to be imaged so that either this area remains in the physical centre of rotation of the suspension arm in the imaging means or it is left outside the centre of rotation. In the latter case the physical supporting point of the suspension arm in the imaging means can be moved along a path with the shape of an arc by simultaneously turning the suspension arm so that the beam constantly hits the tangent of the curved path of the supporting point in the perpendicular direction, and thus the area to be imaged is the centre of rotation of the curved path. There are also solutions in which the suspension means of the radiation source and the detector are provided with different degrees of freedom to move and solutions in which the radiation source is switched off during one imaging session, moved to another starting point and radiation is started again by rotating the imaging means in the previous direction or in the opposite direction. There are also solutions in which the beam sweeps across the area to be imaged in one direction and returns in the opposite direction and solutions in which the linear direction of movement of the rotation centre of the imaging means is turned during the imaging session.
One restriction of the prior art solutions is that the object itself imposes limitations on the ways in which it can be imaged by these methods. The layer thickness to be projected onto the detector is influenced by the turning angle of the beam in the object to be imaged, the angle being partially dependent on the anatomy of the object to be imaged. For example, when a narrow beam is used for imaging a straight object, the result may be a nondesirable X-ray image.
The problem associated with the use of a wide beam is that it is necessary to use radiation detectors with a large surface area. The need for a large surface area restricts the use of digital detectors which are relatively expensive, their price being proportional to their surface area.
Considering simplicity of the suspension structure of the imaging means, it would be preferable to keep the detector perpendicular to the beam during the whole imaging session, but if a beam of the same size as the object to be imaged is used, the resolution of the imaging result will not be optimal in this case. When a narrow beam is employed, the error resulting from this is insignificantly small.
In narrow beam tomographic methods, in which the imaging process is based on the sweep of a narrow beam across the object and which thus allow the use of detectors with a small surface area, the use of digital detectors is also economically feasible. In fact, it would be desirable to find new solutions to the problems related to the general limitations of the prior art narrow beam tomography methods.
Devices intended for different tomographic methods have conventionally been produced for a certain tomographic method. However, the present trend is to develop solutions which allow to use one device for various purposes, i.e. the goal is to be able to use the same device in different tomographic methods and for imaging different projections. For example, forming of beams of different sizes and defining their size as desired before supplying them to the detector have proven to be challenging problems. Furthermore, production of high-quality tomographic images requires that the detector and/or radiation source of the device should be provided with sufficiently many degrees of freedom to move so that the detector can be kept in the correct direction with respect to the object to be imaged in all imaging modes.
An object of the present invention is to provide a solution which allows to image layers of the desired thickness and shape from different objects by means of one preferred device without complex collimator structures which impose limitations on the beam and without structures which are difficult to implement and comprise several degrees of freedom to move. Another object of the invention is to produce a tomographic effect so that the imaging means can follow the anatomy of the object to be imaged without having to make compromises between the thickness of the layer to be imaged and the optimal imaging path, i.e. a path which is perpendicular to the object to be imaged.
Furthermore, when the same device can be used for implementing different imaging modes according to the invention, investment in imaging sensors based on modern digital technology becomes more profitable, which lowers the threshold of introducing them. Digital technology facilitates the doctors"" work, for example, since it does not only allow the doctors to produce better images than earlier and thus to make more accurate diagnoses, but also to store the images and manage the images in electronic form, together with all other documents related to the patient.
To achieve the objects of the invention, a new way of producing a tomographic effect has been developed for tomographic methods based on the use of a narrow beam. The invention is based on the idea that the direction of rotation of the beam is changed in the object to be imaged during an imaging scan, and thus creation of the tomographic angle (different directions from which the beam penetrates the object to be imaged), which affects the thickness of the layer to be imaged, can be arranged as a separate function. This can be preferably implemented e.g. in a method known per se, in which the object to be imaged is held in the physical centre of rotation of the imaging means, and the suspension arm of the imaging means, at the opposite ends of which the radiation source and the detector are located, is moved along the object to be imaged (linear movement) as the detector, e.g. a film, is moved in the opposite direction with respect to the movement of the arm at a speed which corresponds to the beam speed in the area to be imaged multiplied by magnification. In that case the centre of rotation of the arm moves at the speed of the arm""s linear movement regardless of the direction in which the suspension arm of the imaging means is turned (the tomographic effect is produced by turning the arm). By rotating the suspension arm to and fro in the desired manner during the scanning motion a tomographic image with the desired thickness dependent on the tomographic angle can be produced even from a straight object.
The method of the invention can be applied in several prior art methods, e.g. in the method disclosed in the above-mentioned U.S. Pat. No. 4,039,837 for imaging a curved object. In that case the suspension arm of the imaging means is moved according to the prior art so that the centre of rotation is at the object to be imaged and the beam is perpendicular to the tangent of the curve of the object, and thus a narrow beam tomographic image which is known from panoramic imaging and dependent on the radius of curvature of the object is obtained. If the radius of curvature is large, the tomographic angle will be small when a narrow beam is used, and as a result, the layer to be imaged will be too thick. Instead, by changing the direction of rotation of the suspension arm of the imaging means according to the invention, the tomographic effect can be increased, and thus the thickness of the layer to be imaged can be chosen as desired. The invention allows tomographic imaging of an anatomy of any shape, i.e. the object may be curved in one or two directions, straight or a combination of these.
More precisely, characteristic features of the invention are described in the characterizing parts of the appended independent claims.
In the following the invention and its application will be described by examples with reference to the accompanying drawings.